The frontlines of Yemen's messy civil war have barely moved after weeks of Saudi air strikes and fighting between rival factions, but behind the scenes there are faint signs of shifts that could alter the balance on the ground.

At stake is the stability and political future of a country that is home to Al Qaeda's most active international wing, has become a theatre of rivalry between top regional powers Saudi Arabia and Iran, and sits on main shipping routes.

But as the conflict continues and the impoverished country hurtles deeper into a humanitarian catastrophe, Yemenis warn that the longer the fighting lasts, the harder it will be to restore meaningful state control.

"By reading the map of all Yemen's battles and military clashes, both the air strikes and the internal Yemeni battles, one can see that the front is in the same place it was the day it all started, and neither side could push its advantage," political activist Bushra Al-Maqtari wrote in the Al-Araby Al-Jadeed newspaper on Sunday.

The Houthi militia and allied army units loyal to former president Ali Abdullah Saleh still hold central and southern areas they seized in late March and early April, but have failed to quash resistance from forces backing the exiled government.

Meanwhile, several tribes and army units that had avoided choosing sides have come out in favour of the Riyadh-based President Abd-Rabbu Mansour Hadi over the past week, but appear to have made little progress in turning the tide of battle.

"The Houthis attack and the tribesmen confront them -- one side will take control over a position but by the next day the other side has taken it back," said a tribal sheikh in the central desert province of Marib.

All factions involved and outside powers have said they want talks for a political solution, but, with the military outcome still in the balance, no side looks ready yet to make the concessions needed to allow dialogue.

The Houthis have close relations with Iran and Riyadh fears Tehran wants to use the group to project might into Saudi Arabia's backyard, undermining its security.

STRIKES AND STALEMATE

The Houthis and Saleh are fighting to hold the stalemate, knowing that international support for Hadi's government in Riyadh, and for the Saudi air strikes, will start to fade as Yemen disintegrates further into a failed state.

They appear to believe the Saudis and Hadi would then be put under increasing pressure to cut their losses and make a deal that would recognise the Houthis' kingmaking position and allow Saleh to retain his influence, say analysts.

The Saudis, in contrast, believe their strategy is slowly paying dividends. Air strikes have put pressure on Saleh's army loyalists, destroyed much of their military equipment and cut them and the Houthis off from outside support.

They aim to use that pressure, alongside political back-channels to tribal leaders, politicians and army generals, to isolate Saleh and persuade his supporters to defect to Hadi, leaving the Houthis militarily exposed and overextended.

"Before, they (the Houthis) were just giving lip service to the idea of sharing power -- now that they realise they can't win on the battlefield, they will have to do more than that," said a Yemeni politician critical of the militia who asked not to be named for security reasons.

SWITCHING SIDES

Recent fighting has focused on four main fronts.

In the southern port of Aden, the Houthis and an army garrison loyal to Saleh are fighting local militias backed by Saudi air strikes. In Taiz, Yemen's third city, they have been locked in street battles with Sunni Muslim Islamist fighters.

At the same time, the Houthis and Saleh's forces have been fighting tribes, Islamists and local militias in Marib province, east of the capital Sanaa, and in Shabwa, northeast of Aden.

The tribes and local fighters have light weapons and little access to outside supplies except for what Saudi warplanes have air-dropped. Their opponents benefit from close ties to the country's regular army and have tangled but clear supply lines through newly won provinces.

"The Houthis are getting their supplies from army forces' stores in this province and others, and they have enough food to last them months," said Jamal Al-Awlaqi, a tribal fighter against the Houthis in Shabwa.

For now, dramatic changes on the ground seem likely to come via back-channels, rather than through any sudden shift in the military equation. Several defections of army units to Hadi last week suggested a Saudi effort to win influence.

No further defections have been publicly announced, and Saleh last week made a defiant statement that he would stay in Yemen rather than flee abroad. Persuading his generals otherwise, with bribes, threats or blandishments, may now be Saudi Arabia's best shot at winning its Yemen campaign. - Reuters